Conventionally, as a semiconductor memory device, a NAND flash memory has been developed. As a NAND flash memory, a rocket-shaped memory has been proposed. In the rocket-shaped memory, the upper portion of the floating gate electrode is projected upward from the upper surface of the device isolation insulating film. An IPD (inter-poly dielectric) film is formed so as to cover this projected portion of the floating gate electrode, and a control gate electrode is provided thereon. Thus, the control gate electrode is inserted into the gap between the floating gate electrodes so that the floating gate electrode is surrounded from three directions. This can enhance the control power of the control gate electrode on the floating gate electrode.
However, the miniaturization of the NAND flash memory results in narrowing the gap between the floating gate electrodes. This decreases the embeddability of the control gate electrode and causes the problem of the generation of voids. If a void is generated in the control gate electrode, the potential fails to be sufficiently applied to the portion of the control gate electrode below the void. This decreases the reliability of the operation.